I took apart the fuel cap and you are correct it only vents one way. Why?

I disabled the one way venting system so now it vents both ways and it pulls air around the fuel filler "moat" Hopefully that will allow the main tank to vent enough so that it's not forcing it up subtank vent. Now the question is how to make the subtank vent so that water can't get in from rain.

I think I can form a wall with some JB weld and shove some gas foam down there to keep most of the water at bay.

post9-are you quite sure that's a vent? forgive me for asking, but to my limited experence, it looks like a drain for sloppy fueling/rain water. i have wrestled with aug tank problems too, in my case a st1100 with a sampson aux. fender type tank-got it used with NO Installation instructions. the problem sounds like what i had. the cause in my case was that the fuel pump raised the pressure in the tank, and forced fuel out the aux fuel tank vent line.(fist go round it was ventedto the air right above the r. tire. and for a 1/8 inch line, it delevered an alarming amount of fuel to my rear tire) the fix for me was to reroute the sub's vent line into the main, and at the highest point on the main.

I took apart the fuel cap and you are correct it only vents one way. Why?

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Letting fuel fumes out is probably prohibited by emissions regs, except via a charcoal canister. That's just a guess, though.

I disabled the one way venting system so now it vents both ways and it pulls air around the fuel filler "moat" Hopefully that will allow the main tank to vent enough so that it's not forcing it up subtank vent. Now the question is how to make the subtank vent so that water can't get in from rain.

I think I can form a wall with some JB weld and shove some gas foam down there to keep most of the water at bay.

Click to expand...

Another option would be to run the vent line into the main tank through the fuel filler cap. Any spew would just go into the tank. Routing the vent line to the front of the tank would prevent length problems when the filler cap hinged forward to open.

Or, you could [edit:never mind, you'd just dump fuel with this idea.]
There might be problems with both suggestions, but they might also be helpful.

post9-are you quite sure that's a vent? forgive me for asking, but to my limited experence, it looks like a drain for sloppy fueling/rain water. i have wrestled with aug tank problems too, in my case a st1100 with a sampson aux. fender type tank-got it used with NO Installation instructions. the problem sounds like what i had. the cause in my case was that the fuel pump raised the pressure in the tank, and forced fuel out the aux fuel tank vent line.(fist go round it was ventedto the air right above the r. tire. and for a 1/8 inch line, it delevered an alarming amount of fuel to my rear tire) the fix for me was to reroute the sub's vent line into the main, and at the highest point on the main.

Click to expand...

You are correct it wasn't a vent it was a drain for fuel spills. I hadn't thought about rain water though. Doh! I repurposed it for the vent for the subtank, and it seemed like a convenient spot for it.

I thought there must be a check valve of some kind that would allow air to pas but not fuel and put it in the line.

Anyways if I'm going to keep that line as a vent I will ned to be careful about water getting in there.

Letting fuel fumes out is probably prohibited by emissions regs, except via a charcoal canister. That's just a guess, though.

Another option would be to run the vent line into the main tank through the fuel filler cap. Any spew would just go into the tank. Routing the vent line to the front of the tank would prevent length problems when the filler cap hinged forward to open.

Or, you could [edit:never mind, you'd just dump fuel with this idea.]
There might be problems with both suggestions, but they might also be helpful.

Click to expand...

Not sure how I'd go about this. I want to keep that fuel cap vent open so that pressure can't build in the tank

Another option would be to run the vent line into the main tank through the fuel filler cap. Any spew would just go into the tank. Routing the vent line to the front of the tank would prevent length problems when the filler cap hinged forward to open.

Click to expand...

Not sure how I'd go about this. I want to keep that fuel cap vent open so that pressure can't build in the tank

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I was thinking that you'd take your vent line from the sub-tank and run it underneath the main tank, all the way to the front, then up, over the top and to the filler cap. You'd have to modify the cap to accept the vent hose while retaining the water seal, and retaining proper cap vent operation. I was thinking something like a straight (not 90º!) barb fitting bolted/glued into a hole drilled all the way through the cap.

I don't know if your cap has enough space to drill a hole, install some sort of pipe/tube (to prevent fuel seepage, and retain a vapor-tight seal), and glue/weld/bolt a fitting on the top. That's a lot of stuff to put into a fairly small space, and this seems like the biggest challenge to what I've suggested.

On the positive side, you would retain all the original characteristics of the positive-pressure tank, and (hopefully) retain the action of the tank vent. Your sub-tank vent line would be above the max fuel level in places where it's exposed. Areas where it's below max fuel level could be protected fairly easily, for example by using fuel line as chafe gear.

You'd also obtain that awesome old-school race-bike look, with a fuel tank cap with a vent line loop sticking out the top.

I hope that clarifies what I'm thinking about, and good luck with it no matter how you end up dealing with it.

things are a bit clearer now-I always say that before sticking my foot in mouth. the expelled fuel over flow is coming from the slop drain( or the fliptop cover), isn't it? clearly the main tank enjoys an elevated operating pressure. ( surprisingly this is normal and part of the design of the bike)- and by extension the sub too! end result is that the gas is blown out the sub and into the slop drain -and if i'm right about that, only rerouting the subs vent line INTO the main will fix it.( i'm not sure if routing to the lower part of the main will do anything for you, except to drastically slow down refueling stops.) this would be the exact same thing I ran up against. piping slightly different, but same problem. if you can't insert a vent into the top section of the main, then you might have to silver solder a tube that goes from out side bottom(hidden from view)- thru the tank to someplace reasonably close to the top. I believe you mentioned that the vent line is an AN#6 this would be the time to make the sub's vent bigger, as well.

I was thinking that you'd take your vent line from the sub-tank and run it underneath the main tank, all the way to the front, then up, over the top and to the filler cap. You'd have to modify the cap to accept the vent hose while retaining the water seal, and retaining proper cap vent operation. I was thinking something like a straight (not 90º!) barb fitting bolted/glued into a hole drilled all the way through the cap.

I don't know if your cap has enough space to drill a hole, install some sort of pipe/tube (to prevent fuel seepage, and retain a vapor-tight seal), and glue/weld/bolt a fitting on the top. That's a lot of stuff to put into a fairly small space, and this seems like the biggest challenge to what I've suggested.

On the positive side, you would retain all the original characteristics of the positive-pressure tank, and (hopefully) retain the action of the tank vent. Your sub-tank vent line would be above the max fuel level in places where it's exposed. Areas where it's below max fuel level could be protected fairly easily, for example by using fuel line as chafe gear.

You'd also obtain that awesome old-school race-bike look, with a fuel tank cap with a vent line loop sticking out the top.

I hope that clarifies what I'm thinking about, and good luck with it no matter how you end up dealing with it.

Click to expand...

I see what you are saying now. I get it and I like it.

Couple issues with your plan, the cap has that little flapper that covers the key hole I'd have to get rid of. Also there are internals that I would have to seriously dodge to keep the key lock functional. Plus the cap is fairly complex with the venting system as I discovered yesterday. I'll have another look and see if I can figure out a solution. Thanks!

things are a bit clearer now-I always say that before sticking my foot in mouth. the expelled fuel over flow is coming from the slop drain( or the fliptop cover), isn't it? clearly the main tank enjoys an elevated operating pressure. ( surprisingly this is normal and part of the design of the bike)- and by extension the sub too! end result is that the gas is blown out the sub and into the slop drain -and if i'm right about that, only rerouting the subs vent line INTO the main will fix it.( i'm not sure if routing to the lower part of the main will do anything for you, except to drastically slow down refueling stops.) this would be the exact same thing I ran up against. piping slightly different, but same problem. if you can't insert a vent into the top section of the main, then you might have to silver solder a tube that goes from out side bottom(hidden from view)- thru the tank to someplace reasonably close to the top. I believe you mentioned that the vent line is an AN#6 this would be the time to make the sub's vent bigger, as well.

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Yes you are correct, the vent from the subtank is now the repurposed slop drain. And it ain't workin'

I'm hesitant to weld on the tank, mainly out of fear that it will screw up the liner and I don't want to have to deal with removing crispy POR15 from the tank. I guess if someone at POR said it's no big deal, I would weld in a "straw that exited near the top of the tank and problem solved.

The way the cap works achieves two things that your system is not:
1) Slow and little fuel spillage in the case the bike falls on its side.
2) Reduce fuel evaporation.

For normal operation, the auxiliary tank needs no vent.
After you run it dry, a manual vent valve will suffice to re-fill it by gravity.

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I think the manual vent for refilling is the best option at the moment. As you stated, the subtank does not need to be vented until you are refueling and the subtank needs to displace the air so that gravity can do its thing.

With 4 gals in the main tank and 1.2 in the subtank, I do have quite a bit of range. The effort to crack open a vent for 30 seconds and allow the subtank to fill and then close is fairly minor in the course of owning a bike like mine with so many crazy attributes to start with.

I am not totally satisfied with the tank welding and modifying I did. I have given consideration to finding another tank and redoing it. Perhaps when the winter teardown for painting and powdercoating happens. It's a couple hundred for a new tank and I can properly plan the location of the fuel supply and venting now knowing what I am in store for.

Just in case you are all wondering what the hell I am talking about, here's the maiden voyage a few weeks ago, since put 200 miles on it. The subtank sits in back, and around the other side of the shock.

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I think the manual vent for refilling is the best option at the moment. As you stated, the subtank does not need to be vented until you are refueling and the subtank needs to displace the air so that gravity can do its thing.

With 4 gals in the main tank and 1.2 in the subtank, I do have quite a bit of range. The effort to crack open a vent for 30 seconds and allow the subtank to fill and then close is fairly minor in the course of owning a bike like mine with so many crazy attributes to start with.

I am not totally satisfied with the tank welding and modifying I did. I have given consideration to finding another tank and redoing it. Perhaps when the winter teardown for painting and powdercoating happens. It's a couple hundred for a new tank and I can properly plan the location of the fuel supply and venting now knowing what I am in store for.

Just in case you are all wondering what the hell I am talking about, here's the maiden voyage a few weeks ago, since put 200 miles on it. The subtank sits in back, and around the other side of the shock.

You could run that manual vent tube into a vented smaller transparent tank, so you can see when to stop before you make a mess on the floor of the gas station.
A better alternative: For industrial piping systems, there are automatic vents that are located at highest points, which blow out any air in the pipes but close when the fluid reaches them.
Check out how they work:http://www.spiraxsarco.com/pdfs/sb/s31_06.pdf

Another thing about which I have been thinking for your solution: rain will eventually find a path around the cap, into the tank and into the subtank (lowest point of the tank system).
If not, some condensation will happen inside anyway.
You will need a way to drain that water from the lowest point, as I don't think that the pump and injectors will be able to get rid of it.

My bike has a carburetor and a cockpit with reserve.
Once in a while I drain the bowl and the reserve into a glass container, collecting an important amount of water.

Couple issues with your plan, the cap has that little flapper that covers the key hole I'd have to get rid of. Also there are internals that I would have to seriously dodge to keep the key lock functional. Plus the cap is fairly complex with the venting system as I discovered yesterday. I'll have another look and see if I can figure out a solution. Thanks!

Click to expand...

Yeah, this idea hinges on there being enough space in the cap to drill a reasonable-sized hole, and insert a reasonable-sized bit of tubing. It doesn't have to be big, but as you say, there's a lot of stuff in a fairly small space there.

I'll take a short moment to comment on joexr's suggestion that you bolt a bulkhead fitting onto the tank. Your fuel filler hole isn't huge, but it might be big enough to get tools in/out (or find a small-handed friend/neighbor-kid). If you can do that, then my idea gets even easier, because you just bolt a bulkhead fitting directly in front of the gas cap.

Yeah, this idea hinges on there being enough space in the cap to drill a reasonable-sized hole, and insert a reasonable-sized bit of tubing. It doesn't have to be big, but as you say, there's a lot of stuff in a fairly small space there.

I'll take a short moment to comment on joexr's suggestion that you bolt a bulkhead fitting onto the tank. Your fuel filler hole isn't huge, but it might be big enough to get tools in/out (or find a small-handed friend/neighbor-kid). If you can do that, then my idea gets even easier, because you just bolt a bulkhead fitting directly in front of the gas cap.